Various bone screws are known in the field. For the stabilisation of a bone fragment multiple screws are normally used to fix the bone fragment to a bone. If only one screw is inserted, the bone fragment could rotate around the axis of the screw and would therefore not exhibit the necessary stability. However, in many indications only one screw may be placed into the bone fragment. For example, in femoral head fractures, only one screw may be used to fixate the femoral head to the femoral shaft. However, when using a single screw, there is a risk that the femoral head rotates around the screw axis, thereby losing the fracture reduction.
Hip fractures are a common injury. The number of annual hip fractures is increasing worldwide. Especially the elderly population suffers from hip fractures. Due to the increasing number of elderly people, the annual number of hip-fractures is increasing fast. One reason for a hip fracturing is osteoporosis. With rising age, the bones become more brittle and easily fracture after a fall.
There are many types of hip-fractures, which need an individual treatment. For example a femoral neck fracture in a patient with bad cartilage on the joint surfaces is preferably treated with a total hip implant, completely replacing the joint. Fractures in patients with good quality cartilage and younger patients preferably are treated with methods that do not replace the entire joint.
Several methods of treatment are available depending on the location of the fracture, the number of bone fragments, the quality of the bone, and the size of the bone. Treatment methods vary from the insertion of two or more cannulated screws to the insertion of intramedullary nails to stabilize the bone fragments.
Another common method is the implantation of the dynamic hip screw, or sliding screw fixation. The dynamic hip screw consists of a plate which is fixated to the lateral cortex of the femoral bone, and a large screw extending from the plate into the femoral head. The screw can telescope into the plate, a so-called dynamic fixation. The dynamic fixation allows the femoral head to set itself against the femoral shaft, and the implant to shorten accordingly.
A disadvantage of the dynamic hip screw is that the screw may withstand tensile loads and compression loads, but cannot withstand torsional moments. If a rotation occurs, the fracture reduction is lost. Further, the rotation may also disrupt the blood supply to the head of the femur and cause so-called avascular necrosis. The head fragment is thereby cut off the necessary blood supply to heal. In case of avascular necrosis, in a next surgical intervention the implant is removed and replaced by a total hip prosthesis.
Often an extra anti-rotation screw is placed above the dynamic hip screw implant to overcome this problem. The disadvantage of the additional screw is that the necessary bone stock must be available and the telescoping of the screw into the plate might be compromised. Especially in small stature patients the femoral neck may simply be too small for the application of an extra anti-rotation screw.
Other available designs to secure the implant from torsional moments comprise two parallel screws extending into the femoral head, wherein both screws telescope in the bone plate. These designs do inhibit rotation of the head fragment, but are large in size.